1. Field of the Invention
The present invention relates to an ink composition for forming a dye layer, a constituent layer of a heat transfer printing sheet, and to a heat transfer printing sheet using the ink composition.
2. Background Art
Heretofore, heat transfer printing methods have widely been used as printing methods for forming, on image-receiving sheets, images with gradation, or monotone images such as characters and symbols. Of the heat transfer printing methods, printing methods of sublimation transfer type and of hot-melt transfer type are now used extensively.
The heat transfer printing method of sublimation transfer type is as follows: a heat transfer printing sheet comprising a substrate sheet carried on a dye layer which incorporates sublimation dye dispersed or dissolved in a binder resin is superposed on an image-receiving sheet, and energy is applied to the heat transfer printing sheet by a heating means such as a thermal head or laser to sublime the dye to transfer it to the image-receiving sheet, or to diffuse the dye to migrate it to the image-receiving sheet, thereby forming an image on the image-receiving sheet. On the other hand, the heat transfer printing method of hot-melt transfer type is as follows: a heat transfer printing sheet comprising a solid ink composition composed of a waxy binder and a coloring material such as a pigment is superposed on an image-receiving sheet, and energy is applied to the heat transfer printing sheet by a heating means such as a thermal head or laser to melt the solid ink composition; the melted ink composition is thus transferred to the image-receiving sheet and produces thereon an image.
In the heat transfer printing method of sublimation transfer type, it is possible to control the amount of a dye to be transferred or migrated to an image-receiving sheet to form thereon one dot by changing the amount of energy that is applied to a heat transfer printing sheet. This method can therefore successfully produce a full-color image that is excellent in gradation and that has high quality comparable to that of conventional silver-salt photographic images. Owing to this advantageous feature, the heat transfer printing method of sublimation transfer type is now attracting attention, and being employed in various fields as a means for recording information.
On the other hand, one important feature that is required for heat transfer printing sheets is separability from image-receiving sheets. In a heat transfer printing method, a heat transfer printing sheet is superposed on an image-receiving sheet, and heat is applied to the heat transfer printing sheet to thermally transfer an image to the image-receiving sheet. Therefore, to attain the smooth transfer of an image, it is essential that the heat transfer printing sheet and the image-receiving sheet be easily separated from each other after the heat transfer printing of an image is completed. Moreover, before and after the heat transfer printing process, the heat transfer printing sheet and the image-receiving sheet are usually carried in the superposed state. Therefore, to prevent the blocking of the two sheets, it is required that the two sheets be easily separable. In particular, in the case where information that is a combination of characters, graphics and images is thermally transferred from a heat transfer printing sheet to an image-receiving sheet having no ink-receiving layer, these two sheets tend to stick to each other while the heat transfer printing of the information is conducted, depending on the material for the substrate of the image-receiving sheet.
To improve the separability of heat transfer printing sheets from image-receiving sheets, there has been proposed a heat transfer printing sheet comprising a dye layer that contains a releasing agent such as silicone. In such a heat transfer printing sheet, however, the compatibility of the releasing agent such as silicone and a binder resin used for the dye layer is not good, so that the releasing agent tends to separate from the binder resin and migrates to the surface of the dye layer. For this reason, even this heat transfer printing sheet cannot be smoothly separated from an image-receiving sheet after an image is thermally transferred to the image-receiving sheet.
To further improve the separability of heat transfer printing sheets from image-receiving sheets, there has been proposed a heat transfer printing sheet comprising a dye layer that contains as a binder resin a graft copolymer having release properties, obtained by graft-copolymerizing a compound having release properties with a polymer. In addition, Japanese Laid-Open Patent Publication No. 67182/1998 describes a heat transfer printing sheet capable of showing improved separability from an image-receiving sheet regardless of the material for the substrate of the image-receiving sheet. In this heat transfer printing sheet, a phosphoric ester is incorporated as a releasing agent into a dye layer, a constituent layer of the heat transfer printing sheet. However, phosphoric esters can react with certain types of dyes when they are mixed, and, as a result, the dyes often undergo change in color. Thus, from the viewpoint of long-term storage stability, there is yet room for improvement even in this heat transfer printing sheet comprising a dye layer that contains a phosphoric ester and a dye.
We found that, in an ink composition comprising a phosphoric ester and a dye, if the phosphoric ester is composed of an acid-type phosphoric ester and a neutralized-type phosphoric ester, the phosphoric ester and the dye hardly react with each other, so that the long-term storage stability of the ink composition is excellent and the discoloration or fading of a dye layer, a constituent layer of a heat transfer printing sheet, formed by using the ink composition is minimized. We also found that a heat transfer printing sheet comprising a dye layer formed by the use of the above ink composition shows improved separability from an image-receiving sheet regardless of the material for the substrate of the image-receiving sheet and can produce an excellent image on the image-receiving sheet. The present invention was accomplished on the basis of these findings.
Namely, an object of the present invention is to provide an ink composition for forming a dye layer in which a dye shows excellent long-term storage stability and which can impart, to a heat transfer printing sheet, improved separability from an image-receiving sheet. Another object of the present invention is to provide a heat transfer printing sheet using this ink composition.
The first embodiment of the present invention is an ink composition for forming a dye layer, a constituent layer of a heat transfer printing sheet. This ink composition comprises a sublimation dye, a binder resin, a phosphoric ester and a solvent, wherein the phosphoric ester is composed of an acid-type phosphoric ester and a neutralized-type phosphoric ester.
The second embodiment of the present invention is a heat transfer printing sheet comprising a substrate sheet, and dye layers of one or more colors formed on one surface of the substrate sheet, wherein at least one of the dye layers is formed by the use of an ink composition according to the first embodiment of the present invention.